1. Field of Invention
The present invention relates to the field of marine seabed data acquisition, which may be seismic or electromagnetic data. More specifically, the invention relates to apparatus and systems, and methods of using same for seabed seismic and/or electromagnetic data acquisition using, among other features, one or more autonomous seabed sensor units.
2. Related Art
Seabed seismic acquisition aims to capture the acoustic and elastic energy that has propagated through the subsurface. This energy may be generated by a surface source such as an airgun or airgun array, also known as a source array. The airgun array produces a pressure signal that propagates through the water column into the subsurface. Here acoustic and elastic waves are formed through interaction with the geologic structure in the subsurface. Acoustic waves are characterized by pressure changes and a particle displacement in the direction of which the acoustic wave travels. Elastic waves are characterized by a change in local stress in the sediment and a particle displacement, which is essentially in the same plane as the wavefront. Acoustic and elastic waves are also known as pressure and shear waves. Shear waves may not propagate in water. Acoustic and elastic waves are collectively referred to as the seismic wavefield.
The structure in the subsurface may be characterized by physical parameters such as density, compressibility, and porosity. A change in the value of these parameters is referred to as an acoustic or elastic contrast and may be indicative of a change in sedimentation, including sediment layers, which may contain hydrocarbons. When an acoustic or elastic wave encounters an acoustic or elastic contrast, some part of the waves will be reflected back to the surface and another part of the wave will be transmitted into deeper parts of the subsurface. The elastic waves that reach the seabed will not propagate back into the water column and hence may only be measured by motion sensors (measuring displacement, velocity, or acceleration, such as geophones, accelerometers, and the like) located on the seabed. The pressure waves are usually measured by hydrophones.
The measurement of both acoustic and elastic waves at the seabed may be used to create a detailed image of the subsurface including a quantitative evaluation of the physical properties such as density, compressibility, porosity, etc. This is achieved by appropriate processing of the seismic data.
The seismic wavefield at the seabed may be measured by sensor units that contain one or more hydrophones and one or more displacement sensors (for example: geophones or accelerometers). These sensor units typically also contain the electronics needed to digitize and record these signals. In one known embodiment, the sensor units are connected to a seabed seismic cable, which again is connected to a recording instrument on a surface vessel/survey vessel or other surface facility such as a platform. The seabed seismic cable provides electric power and the means for transferring the recorded and digitized seismic signals to the recording instrument. A deployment vessel (or other means such as seabed ploughs or seabed tractors) deploys the seabed seismic cable on the seabed. The cable may be deployed on the seabed surface or trenched below the seabed surface. In the latter case a trenching apparatus such as water jet, plough or similar, is used to create a narrow trench in the seabed of the required depth (typically 50 cm-1 m) in which the cable with inventive sensor units is placed.
In another known embodiment, the seabed sensor unit may contain sensors, digitizing electronics, a battery, memory and clock. The sensor unit is placed on the seabed either by an autonomous or remotely operated underwater vehicle (ROV), or by any other means of transport such as conveyor belt system or simply dropped from the sea surface. The seabed sensor unit continuously records seismic data for the duration of deployment. The batteries and memory that are included in the seabed seismic sensor unit must be of sufficient capacity for the expected length of seismic survey period (typically 4 weeks or more). In current practice, such as exemplified by published patent application WO2005071442, once the seismic survey is completed, the seismic sensor units, or at least the portion thereof containing the data are physically retrieved. This is usually achieved by ROV or simply by use of pop-up buoys that brings the sensor unit to the surface. The recorded data is transferred to a main computer system and the batteries are recharged for a next deployment. One advantage of seabed seismic sensor units is flexibility. In principle they may be placed on the seabed at any desired location or in any desired pattern. However, deployment of seabed seismic sensor units is relative slow and units are quite costly, and for these reasons the placement of seismic sensor units on the seabed is usually sparse with several 100 m between two sensor units in any direction. The sensor units may be positioned in regular lattice-like patterns or in pseudo-random patterns.
One particular application of seismic data acquisition is referred to as “time-lapse” or 4D seismic. In time-lapse seismic, a seismic image of the subsurface is made at two or more instances separated by time (lapses). A comparison of these images is used to infer changes in subsurface properties that may be tied to, for example, the production of hydrocarbons, or the injection of water or gas. The time-lapse seismic method is well established and documented for pressure wave recordings in which the seismic measurements are made with hydrophones that are built into seismic streamers towed behind a seismic vessel. The first seismic survey is usually referred to as the “baseline” survey, while any subsequent repeat surveys are usually referred to as monitor surveys. In order to minimize any artifacts in the differences between seismic images from successive lapses, the monitor surveys are usually (but not necessarily) acquired with identical measurement configurations. In addition, vessel, streamer, and/or source array steering may be used to reposition the seismic source and seismic sensors (hydrophones) to the same locations as on the baseline survey. Unlike conventional systems in which the accuracy of the hydrophone locations degrades between acoustic positioning sensor units, Q-Marine technology, available from WesternGeco LLC, delivers consistent accuracy down the full length of the streamers. This improved receiver positioning accuracy translates into improved retention of high frequencies in the seismic dataset. And higher frequencies translate into improved vertical and lateral resolution. A survey vessel known as a Q-Technology™ vessel may conduct seismic surveys towing multiple, 1000-10,0000-meter cables with a separation of 25-50 meters, using the WesternGeco proprietary calibrated Q-Marine™ source. “Q” is the WestemGeco proprietary suite of advanced seismic technologies for enhanced reservoir location, description, and management.
For seabed seismic surveying, the application of time-lapse seismic is relative new. This is mostly due to the problem of repositioning the seabed inventive sensor units in exactly the same location as for the monitor survey and ensuring a consistent coupling of the inventive sensor unit with the seafloor sediment. Any changes in coupling at a later deployment or changes in position will be manifest in the seismic image and hence compromise the interpretation of the time-lapse differences. The only accepted solution that has been commercially available to date is using permanently deployed seabed seismic cables. In this case, a seabed seismic cable is not retrieved from the seabed between successive seismic surveys. For safety reasons, usually but not necessarily the seabed seismic cable is trenched into the seabed. This approach has been used on several prospects to date. The problem with this solution is that it is very costly, inflexible and in-situ repair of malfunctioning equipment is not possible without compromising the coupling integrity.
In theory, seabed seismic sensor units may also be used for time-lapse seabed seismic monitoring, but since the seismic sensor units, or at least portions thereof containing the data, must be retrieved after every survey, this approach is impractical for short time-lapses (typical 3 months between each survey). On redeployment the sensor unit has to be put back into its previous position. This is not unachievable with today's technology, but the major draw back is that the soil parameters at its place of origin have likely changed since last installation and hence it will be close to impossible to repeat the same coupling of the sensor unit to the seabed as during the last lapse. This will again introduce artifacts in the time-lapsed seismic image.
Semi-autonomous seabed seismic sensor units are currently commercially available. However, they suffer from several deficiencies—in particular with respect to power consumption. This means that their batteries consume a large volume and weight of the sensor unit. Also, these sensor units must be retrieved from the seabed after each seismic survey in order to use the seismic data that has been collected. Such a sensor unit is described in U.S. Pat. No. 7,124,028.
Another known system is described in published patent application WO 2005071442 A2, which discloses a seabed seismic sensor unit with a permanently deployed base and a retrievable electronics/battery/memory unit.
There has been a long-felt, but as yet unmet need in the marine seabed seismic and electromagnetic data acquisition industries to overcome the limitations of seabed sensor units for time-lapse seismic surveying, in particular in a mode in which the sensor units are left on the seabed between surveys. To date, this is only possible by costly permanently deployed seismic cables on the seabed. The seismic sensor units referred to in WO2005071442 A2 address the time-lapse coupling problem by leaving the node base coupled to the seabed, but only allow data retrieval by removing a substantial part of the seabed node. The present invention addresses and provides solutions to one or more key limitations of seabed seismic cables, seabed seismic sensor units, and electromagnetic sensor units, namely cost, flexibility, continuous power and data transfer. It would be advantageous if electromagnetic (EM) measurements and or surveys could be made using a sensor unit that offers continuous power and data transfer features. It would further be advantageous if sensor units were available having these power and data transfer features and able to collect both EM and seismic data, either simultaneously or sequentially, and/or perform one or more seismic and EM surveys.